BerkeleyGW is a many-body perturbation theory code for quasiparticle calculations in the GW approximation and optical calculations with the Bethe-Salpeter equation (BSE). It is freely available from http://www.berkeleygw.org. Octopus can produce the mean-field input for a BerkeleyGW calculation, if Octopus has been linked with the BerkeleyGW I/O library.
You must first install BerkeleyGW. Then type make library from the top level, which will create needed *.mod files and the library libBGW_wfn.a in the library directory. Then configure Octopus, including in the command line --with-berkeleygw-prefix=$HOME/BerkeleyGW or whatever the appropriate path is to the BerkeleyGW installation. Now you can compile and link as usual.
BerkeleyGW output is enabled by setting Output = berkeleygw in the input file. Other variables in section Output::BerkeleyGW control the output. You will get the following files: VXC, RHO, WFN, vxc.dat, and optionally x.dat, all in the static directory.
The code and its formalism are described in this paper:
Jack Deslippe, Georgy Samsonidze, David A. Strubbe, Manish Jain, Marvin L. Cohen, and Steven G. Louie, "BerkeleyGW: A Massively Parallel Computer Package for the Calculation of the Quasiparticle and Optical Properties of Materials and Nanostructures," Comput. Phys. Commun. 183, 1269 (2012) (http://arxiv.org/abs/1111.4429)